There is a current trend to integrate photonic devices and electronic devices on the same semiconductor substrate. A silicon-on-insulator (SOI) substrate can be used as the supporting substrate for such integration. When photonic devices such as optical waveguides are formed a cladding is provided around the core of the waveguide for confining a light wave propagated along the waveguide. The core material has an index of refraction which is larger than that of the cladding. If silicon is used as the core material of a waveguide, having an index of refraction of about 3.47, the waveguide cladding can be formed of a material having a lower index of refraction. For example, silicon dioxide, which has an index of refraction of about 1.54 is often used as the waveguide cladding.
When a silicon-on-insulator substrate is used as the supporting substrate, the cladding material below the waveguide core can be the buried oxide (BOX) insulator of the SOI substrate, which is again typically silicon dioxide, and the waveguide core can be formed from the silicon above the BOX insulator. The BOX cladding functions to prevent optical signal leakage by evanescent coupling from the silicon waveguide core to a supporting silicon substrate of the SOI structure. However, to prevent such evanescent coupling, the BOX cladding material beneath the waveguide core must be relatively thick, for example, greater than 1.0 μm and often 2.0 μm -3.0 μm thick. When the Box cladding material is thick it inhibits heat flow to the underlying silicon, thus diminishing its effectiveness as a heat dissipater, particularly for CMOS circuits which may be formed on the same substrate. In addition, when certain electronic devices, such as high speed logic circuits, are integrated on the same SOI substrate as photonic devices, the BOX of the SOI substrate must be relatively thin, typically having a thickness in the range of 100-200 nm. Such a thin BOX insulator SOI, while providing a good substrate for the electronic devices, is insufficient to prevent evanescent coupling of the silicon waveguide core to the underlying supporting silicon of the SOI substrate, which causes undesirable optical signal loss. In addition, SOI substrates are relatively expensive and sometimes of limited availability.
Accordingly, non-SOI substrates have also been used to integrate electronic and photonic devices on the same substrate. One technique which may be used to prevent evanescent coupling of an optical device to an underlying non-SOI substrate is described in U.S. Pat. No. 7,920,770. In this patent, a deep isolation trench is etched in the substrate below a fabricated optical device. The etching described provides a trench which is formed in a generally curved shape below the optical device. As noted, an underlying cladding material for a photonic device, such as a waveguide core, must be at least 1 μm thick, and is preferably 2.0 μm-3.0 μm thick. It should also extend at that depth laterally past each side edge of the photonic device by at least 1 μm. However, to meet the cladding depth criteria a curved trench would require a lateral extent past the side edges of a photonic device of greater than 1 μm. The larger the lateral extent of the curved trench beyond the side edge of a photonic device, the greater substrate real estate which must be provided for forming the photonic device. The '770 patent also discloses that an additional optical device fabrication material is provided over the substrate for formation of an optical device.
What is needed is a non-SOI substrate suitable for forming both CMOS and photonic devices which provides a generally rectangular shaped lower cladding, as well as a simplified method of forming photonic devices and underlying cladding. A substrate structure which does not require the presence of an additional optical device fabrication material over a non-SOI substrate is also desired.